With the arrival of broadband era, demand for larger capacities for optical fiber transmission systems has been increasing. In order to implement a large capacity system, it has been common practice to increase the signal speed for each transmission channel by means of time division multiplexing in an electric stage circuit, and to further improve the transmission capacity by means of wavelength multiplexing in an optical stage. Because of speeding up electric circuits, a wavelength multiplexing system based on the channel speed of 10 Gbit/s is widely used, and a system based on 40 Gbit/s channel is on the verge of implementation.
As a line code for wavelength multiplexing of such a high-speed optical signal, a DPSK (differential phase-shift keying) having one bit for the amount of information per symbol, two-bit DQPSK (differential quadrature phase-shift keying), and three-bit D8PSK (differential 8-phase sift keying) draw attention. These line codes feature that “1”/“0” information is transmitted as information on the phase of light, rather than intensity of light (direct detection). This can not only obtain a high receiver sensitivity, but also has advantages of excellent nonlinear strength and the like, so that its study has been recently active. Further, from a sensitizing standpoint, RZ-DPSK (Return-to-Zero DPSK) in which the phase-modulated optical signal is further subjected to pulsed intensity modulation becomes mainstream. As RZ pulse methods, CSRZ-DPSK (Carrier Suppressed Return-to-Zero DPSK) (see Non-Patent Documents 1 and 2) as a new modulation code in which only a phase p is shifted between adjacent pulses is proposed in addition to the conventional RZ modulation.
The reason why these line codes can attain higher receiver sensitivity characteristics than the direction detection is because a balanced optical receiver is used to improve the signal-to-noise ratio. It can improve a receiver sensitivity of 3 dB compared to the direct detection method. For example, in order to receive a phase-modulated signal at the balanced optical receiver in the DPSK receiver, the phase-modulated signal light is passed through MZI (Mach-Zehnder interferometer) in which a difference corresponding to 1 symbol is inserted during propagation delay time of two optical waveguides as shown in FIG. 28(a). Then, it is demodulated to the intensity modulated signal by interference between the optical phase (0 or p) of the previous symbol and the optical phase of the next symbol to output from the two output ports as a signal “1” or “0”. Therefore, as shown in FIG. 28(b), there is a need to mach the carrier frequency and a frequency with which the MZI transmittance becomes the maximum or minimum. FIG. 28(c) shows such a state that the transmittances of carrier frequencies for two ports of output 1 and output 2 are set to maximum (constructive) and minimum (destructive), respectively. In other words, if no phase inversion occurs between adjacent bits of the phase-modulated signal light, the optical signal is output to output 1 as “1”, while if phase inversion occurs, it is output to output 2 as “0”. Then, the output is received by the balanced optical receiver as an intensity modulation signal.
However, as shown in FIG. 29(a), if the carrier frequency and the MZI transmittance do not match, light to be output to the output 1 by nature is leaked to the opposite output 2, or light to be output to the output 2 by nature is leaked to the output 1 as shown in FIG. 29(b), resulting in reduction in signal light intensity and interference between codes.
In general, MZI, composed of an optical waveguide or optical fiber, can adjust an optical phase difference of light that reaches an optical multiplexing point after passing through each optical path by heating heaters formed on two optical paths. Thus, the transmittance can be sifted on an optical frequency axis. Therefore, the application of heat to the heater, i.e., a frequency adjusting terminal, can make the carrier frequency match the maximum or minimum frequency of the transmittance. A driver circuit is used to convert control voltage to current in order to drive the frequency adjusting terminal. Specific means for matching the transmittance of MZI to the carrier frequency is described, for example, in Patent Document 1. In the device described in Patent Document 1, a minute modulation signal (frequency f) is superimposed at optical frequency adjusting terminal through a driver. This minute modulation signal is detected from the output of the optical receiver, and a frequency lock loop is configured that shifts the MZI transmittance with the frequency in such a manner that the output synchronously detected with the minute signal takes zero or a predetermined value. As detection means for the minute modulation signal, a peak detection circuit is used to detect the output amplitude of the optical receiver. As mentioned above, a shift between the carrier frequency and the MZI transmittance causes a reduction in the output amplitude of the optical receiver.
In the meantime, in the method using the peak detection circuit to detect the minute modulation signal, it is indistinguishable as to whether the output port is constructive or destructive upon matching the MZI transmittance and the carrier frequency. Since the MZI transmittance is periodic, if the MZI transmittance is changed on the optical frequency axis, the output port periodically repeats the constructive and destructive states. The repetition cycle of each state is called “FSR (Free Spectral Range). Since the output amplitude of the optical receiver becomes maximum in both states, the output of the synchronous detection circuit becomes zero regardless of whether the MZI output port is set to constructive or destructive as shown in FIG. 30, so that both are indistinguishable. If the output port is wrongly set, since logic “1” or “0” is reversed, normal data reception cannot be performed.
Therefore, in order to distinguish the difference between constructive and destructive of the MZI output port in the device described in Patent Document 1 comprises a second synchronous detection circuit for synchronous detection of an electric signal from an optical receiver circuit, a discrimination circuit for discriminating between positive and negative of the synchronous detection circuit, and an operation point setting circuit for setting an operation point in such a manner that the output of the discrimination circuit is controlled to either of the positive and the negative.
If the phase-modulated signal light is a DQPSK or the like, MZI control becomes further difficult. As means for demodulating the intensity modulation signal from a DQPSK signal in which four phase states (0, p/2, p, 3p/2) exist for each symbol, a structure for placing two MZIs having a 1-symbol delay difference to demodulate a common-mode channel and an orthogonal channel independently is reported as shown, for example, in Non-Patent Document 3 (FIG. 31). In this structure, there is a need to give p/4 and −p/4 phase shifts to short-length optical waveguide (⅛ FSR and −⅛ FSR shifts in terms of frequency). As shown in FIG. 32, the DQPSK deals with four phase states, and four operation points exist for each FSR at which the average amplitude value of the intensity modulation signal after demodulated becomes maximum or the operation point becomes zero. As mentioned above, the DQPSK uses two MZI, and if four operation points exist for each MZI of the common-mode channel and the orthogonal channel, 16 combinations are generated in total. If the demodulated received signal is not subjected to signal processing such as sorting or logic inversion to correct an error in the combinations, one combination of operation points has to be selected from 16 combinations to control the MZI. The MZI of the D8PSK receiver has a structure similar to that in FIG. 31 (Non-Patent Document 4), and the same control is required.    Patent Document 1: Japanese Patent No. 3210061    Non-Patent Document 1: Y. Miyamoto et al., “Novel Modulation and Detection for Bandwidth-Reduction RZ Formats using Duobinary-Mode Splitting in Wideband PSK/ASK Conversion,” J. Lightwave Technol., vol. 20, no. 12, pp. 2067-2078, December, 2002.    Non-Patent Document 2: A. Hirano et al., “Performances of CSRZ-DPSK and RZ-DPSK in 43-Gbit/s/ch DWDMG.652 Single-Mode-Fiber Transmission,” Tech. Dig. on OFC2003, ThE4, pp. 454-455.    Non-Patent Document 3: R. A. Griffin et al., “Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical,” OFC 2002, WX6, 2002.Non-Patent Document 4: Kamio et al., “Study on Delay Detection Differential 8-PSK,” IEICE Technical Report, CS2004-5, p. 23.